In recent years, the application, to next-generation flat panel displays, of light-emitting elements using an organic EL material having characteristics such as thinness, light weight, high-speed responsiveness and direct-current low-voltage driving has been expected. The light-emission mechanism of the light-emitting element is such that an organic compound layer is sandwiched between a pair of electrodes and a voltage is applied thereto, whereby electrons implanted from a cathode and holes injected from an anode are recombined at a light-emitting center in the organic compound layer to form molecular excitons, so that the molecular excitons release energy and emit light when they return to the ground state. It is recognized that the biggest problem in utilizing light-emitting elements using an organic EL material is improving their reliability with the main purpose of extending their light-emitting lifetime.
It is thought that the reasons light-emitting elements using an organic EL element deteriorate are that the constituent material itself deteriorates due to applying an electric field thereto and driving the light-emitting elements, and that the junction state at boundaries of the films configuring the light-emitting elements physically and chemically change. Also, even when the light-emitting elements are simply stored without driving them, they end up deteriorating due to heat, moisture, oxygen, physical shock and sunlight from the outside. Among deterioration resulting from these external factors, deterioration resulting from moisture and oxygen is particularly remarkable.
Dark spots are one example of a poor light-emission phenomenon of light-emitting elements using an organic EL material, and arise due to moisture and oxygen present in the atmosphere. Dark spots are a phenomenon where the light-emission luminance drops locally, and are observed as tiny black spots present in the pixels at the stage immediately after the fabrication of the light-emitting element. It is known that dark spots, which are of a size that is initially imperceptible, grow by driving or storing over a long period of time light-emitting elements using an organic EL material. The main causes of dark spots are moisture and oxygen that penetrate the organic layer through holes (pinholes) formed in the upper electrode. FIG. 2 is an optical microscope photograph of such a dark spot that is frequently observed. FIG. 2 shows the presence of a dark spot 202 in a part (the region encircled by the dotted line in the drawing) of pixels 201 where light-emitting elements using an organic EL material are arranged in a matrix.
FIG. 5 shows the cross-sectional structure of such a light-emitting element, and schematically shows the reason dark spots arise. Reference numeral 501 is a lower electrode, reference numeral 502 is an organic compound layer, and reference numeral 503 is an upper electrode. Dark spots arise due to moisture and oxygen penetrating the organic compound layer through a pinhole 504 that is present in the upper electrode 503 and penetrates the upper electrode 503 as far as the organic compound layer. It is thought that such pinholes arise due to foreign matter, unevenness of the lower electrode, unevenness resulting from crystallization of the organic material (particularly low Tg hole transport material) and crystal grain boundaries.
In an active matrix type light-emitting device, in order to form pixels with thin film transistors (abbreviated below as TFT) and light-emitting elements, several layers of films are formed under the light-emitting element, and there is the potential for foreign matter to adhere to these films during each film-forming process and during conveyance of the substrate. Sometimes foreign matter also adheres to the substrate surface during the film formation of the lower electrode, the organic compound layer and the upper electrode configuring the light-emitting element. Pinholes form in the upper electrode surface if the upper electrode does not completely cover surface unevenness resulting from the foreign matter (e.g., see Non-Patent Document 1).
Non-Patent Document 1
Synthetic Metals, Vol. 91, p. 113 (1997)
Spike-shaped bumps of several nm to several tens nm are present on the surface of an ITO film frequently used as the lower electrode of a light-emitting element. The size of the bumps on the conductive ITO film surface is large, and the bumps can lead to point defects when the ITO and the upper electrode short-circuit, but they can also result in dark spots when they are of a size that will not short-circuit and are not completely covered by the upper electrode.
It is known that, for the above-described reasons, many pinholes are present in aluminum film commonly used as the upper electrode. It is said that there are many pinholes in aluminum film formed by deposition, which is frequently used particularly because it does not damage the organic layer during film formation.
Separation of the upper electrode and the organic layer is recognized as a mechanism where moisture penetrating through the pinholes triggers a drop in luminance (see Non-Patent Document 2), and sometimes the mechanism of deterioration itself is dependent on the material configuring the light-emitting element using an organic EL material, but this mechanism is not completely understood.
Non-Patent Document 2
Applied Physics Letters, Vol. 77, No. 17, p. 2650 (2000)
As one example of a technique for reducing pinholes in the upper electrode, a technique has been disclosed where a low-melting point metal is deposited on a metal electrode layer formed on an organic light-emitting layer of an element using an organic EL material, and the deposited metal is melted to form shield metal that fills in the pinholes in the metal electrode layer (e.g., see Patent Document 1).
Patent Document 1
Japanese Patent Laid-Open No. 2001-52863